Cryogenic liquefaction of normally gaseous materials is utilized for the purposes of component separation, purification, storage and for the transportation of said components in a more economic and convenient form. Most such liquefaction systems have many operations in common, regardless of the gases involved, and consequently, have many of the same problems. One common operation and its attendant problems is associated with the compression of refrigerating agents and the distribution of compression power requirements among multiple gas turbine drivers when multiple cycles, each with a unique refrigerant, are employed. Accordingly, the present invention will be described with specific reference to the processing of natural gas but is applicable to other gas systems.
It is common practice in the art of processing natural gas to subject the gas to cryogenic treatment to separate hydrocarbons having a molecular weight higher than methane (C.sub.2 +) from the natural gas thereby producing a pipeline gas predominating in methane and a C.sub.2 + stream useful for other purposes. Frequently, the C.sub.2 + stream will be separated into individual component streams, for example, C.sub.2, C.sub.3, C.sub.4 and C.sub.5 +.
It is also common practice to cryogenically treat natural gas to liquefy the same for transport and storage. The primary reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate the pipeline under a substantially constant and high load factor. Often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand, thereby enabling future peaks in demand to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a supply source to market when the source and market are separated by great distances and a pipeline is not available or is not practical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significant reduce the specific volume of the gas which in turn requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to -240.degree. F. to -260.degree. F. where it possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas or the like in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, and methane. In the art, the refrigerants are frequently arranged in a cascaded manner and each refrigerant is employed in a closed refrigeration cycle. Further cooling of the liquid is possible by expanding the liquefied natural gas to atmospheric pressure in one or more expansion stages. In each stage, the liquefied gas is flashed to a lower pressure thereby producing a two-phase gas-liquid mixture at a significantly lower temperature. The liquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooled to a storage or transport temperature suitable for liquefied gas storage at near-atmospheric pressure. In this expansion to near-atmospheric pressure, significant volumes of liquefied gas are flashed. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or utilized as fuel gas for power generation.
Obviously, the compressor or compressors employed for compressing the refrigerating agent for a given cycle have operating regimes which are preferred based on turbine/compressor efficiencies and equipment reliability/life expectancy. As an example, the overloading of a given compressor will result in undue wear or damage to that compressor. Unfortunately, a number of operating conditions exist which can fluctuate and affect the loading of individual compressors. Such fluctuations include but are not limited to changes in inlet gas composition, changes in the turbine and compressor efficiency associated with a given refrigerant, changes in climate which affect available turbine horsepower, changes in the return rate of boil-off vapor resulting from ship loading/nonloading conditions, changes attributed to turbine shut-down or start-up (either scheduled or unscheduled) when more than one turbine is used in parallel operation, and changes in the temperature, pressure, flowrate, or composition of the stream to be liquefied resulting from various process operations (fractionating unit, heat exchanger etc.) While individual turbines which drive compressors processing various refrigerants can be protected by such means as speed control mechanisms or the like, such protective means are not a complete answer because changes in the operation of one turbine will change the operation of the entire cryogenic system and can result in the overloading or unbalanced loading of other compressors.